1. Field of the Invention
This invention relates to a water-jet peening method for reducing a residual tensile stress existing in a surface of ordinary metallic materials or a surface layer of parts of internal nuclear reactor equipment and an apparatus for practicing the method and, more particularly, to a water-jet peening method and an apparatus suitable for forming a residual compressive stress in a surface layer by impinging a high-velocity jet flow of the water containing cavities aginst the surface of materials in the water, thereby improving the stressed state thereof.
2. Description of the Prior Art
It is generally known that if a member formed of a metal such as an austenite stainless steel is placed in high-temperature water such as that in a nuclear reactor, intergranular stress corrosion cracking (hereinafter referred to as an abbreviation "IGSCC") occurs in the member in the vicinity of a welded portion thereof. IGSCC is considered to be caused under a condition that three factors, i.e., sensitization of the material, a tensile stress and a corrosive environment, are superposed. Sensitization of a material occurs in such a manner that a Cr carbide precipitates at grain boundaries to form a Cr-depletion layer in the extreme vicinity of the grain boundaries by welding heat or the like so that Cr-depletion zone in the vicinity of the grain boundaries become sensitive to corrosion. A tensile stress is caused by superposition of a residual tensile stress caused by welding or working on a stress caused by an external force. Corrosion is caused by high temperature water containing dissolved oxygen. IGSCC can be prevented by removing one of these three factors.
Japanese Patent Unexamined Publication No. 62-63614 discloses a conventional method as a means for improving a residual stress characteristic. In this method, a high pressure water jetting device having a rotating nozzle capable of jetting a high-pressure liquid jet is inserted into a pipe of a heat exchanger or the like, which is to be improved in a residual stress characteristic, and the high pressure liquid is thereby jetted and impinged upon an inner surface layer of the pipe to cause a plastic deformation in the inner surface layer by dynamic-pressure energy of the impingement of the jet (axial dynamic pressure energy of a jet flow), whereby a residual tensile stress existing originally in the pipe is reduced.
As another residual stress improving means, a surface strengthening method disclosed in Japanese Patent Unexamined Publication No. 59-193215 is known. In this method, an impingement force caused by pressure-collapse of cavities generated in a liquid is applied to a surface portion of a metallic member immersed in the liquid. Cavities are generated by causing ultrasonic vibration of a horn immersed in the liquid as one means thereof, or by jetting a high-pressure high-velocity liquid flow into the liquid as another means thereof.
The conventional process disclosed in Japanese Patent Unexamined Publication No. 62-63614 is effective as a method of improving a residual stress of a pipe inner surface of a heat exchanger or the like. However, this method, in which a liquid flow jetted from a nozzle in atmospheric air is impringed on a surface of a metallic member to peen this surface by impinging energy of the flow, is a method which utilizes impringing dynamic pressure energy of a water jet in atmospheric air and depends upon the water jet output alone. In a case where this method is applied to peening in water, the velocity of a jet flow is considerably reduced before the jet flow reaches a metal surface and it is difficult to achieve the desired peening effect, because the water jet meets a resistance of surrounding water and because the water jet is diffused in the surrounding water. To obtain the same dynamic pressure as that of a jet flow in atmospheric air, water jetting at a super high pressure is required. Thus, the use of this method in such a condition is disadvantageous in terms of performance of a pump and other related components and working cost.
The method disclosed in Japanese Patent Unexamined Publication No. 59-193215, which generates cavities by ultrasonic vibration jetting or high-pressure high-velocity jetting, entails the same problem in the case of high-pressure high-velocity jetting. Ultrasonic vibration jetting is effective in generating cavities, but it is necessary to generate cavities in certain proximity to a predetermined surface portion to be processed, and the effect is therefore restricted by the setting of a horn for generating ultrasonic vibration.